Having thus estimated the size of the earth, Eratosthenes then went on to determine the size of that portion which the ancients considered to be habitable. North and south of the lands known to him, Eratosthenes and all the ancients considered to be either too cold or too hot to be habitable; this portion he reckoned to extend to 38,000 stadia, or 3800 miles. In reckoning the extent of the habitable portion from east to west, Eratosthenes came to the conclusion that from the Straits of Gibraltar to the east of India was about 80,000 stadia, or, roughly speaking, one-third of the earth's surface. The remaining two-thirds were supposed to be covered by the ocean, and Eratosthenes prophetically remarked that "if it were not that the vast extent of the Atlantic Sea rendered it impossible, one might almost sail from the coast of Spain to that of India along the same parallel." Sixteen hundred years later, as we shall see, Columbus tried to carry out this idea. Eratosthenes based his calculations on two fundamental lines, corresponding in a way to our equator and meridian of Greenwich: the first stretched, according to him, from Cape St. Vincent, through the Straits of Messina and the island of Rhodes, to Issus (Gulf of Iskanderun); for his starting-line in reckoning north and south he used a meridian passing through the First Cataract, Alexandria, Rhodes, and Byzantium. The next two hundred years after Eratosthenes' death was filled up by the spread of the Roman Empire, by the taking over by the Romans of the vast possessions previously held by Alexander and his successors and by the Carthaginians, and by their spread into Gaul, Britain, and Germany. Much of the increased knowledge thus obtained was summed up in the geographical work of STRABO, who wrote in Greek about 20 B.C. He introduced from the extra knowledge thus obtained many modifications of the system of Eratosthenes, but, on the whole, kept to his general conception of the world. He rejected, however, the existence of Thule, and thus made the world narrower; while he recognised the existence of Ierne, or Ireland; which he regarded as the most northerly part of the habitable world, lying, as he thought, north of Britain.